Mobile devices that are manufactured for use with the Global System for Mobile Communications (GSM) digital cellular phone technology are designed to work with any mobile communications network service provider. The device requires the use of a subscriber identity module (SIM), referred to as a SIM card, which must be inserted into the GSM device to sign on to the subscriber's service provider network. The SIM card is a small circuit board that contains, among other things, an identifier that identifies the service provider with which the SIM card is allowed to be used. Typically, each service provider, such as AT&T, or Verizon, is assigned their own range of SIM card identifiers for use with their networks.
Most GSM devices are manufactured with a service provider lock that restricts the device to SIM cards for a particular service provider. For example, a mobile device manufactured by Nokia that is marketed by an AT&T service provider may have a lock that restricts the device to SIM cards encoded with identifiers falling within the range of SIM card identifiers assigned for use with the AT&T network.
The method of enforcing the service provider lock may vary from one manufacturer to the next. When a device is manufactured with a service provider lock, the lock is usually based on a code that is stored in the device or derived using an algorithm. However, the codes and/or algorithms may be compromised such that the device may be unlocked and used with SIM cards having identifiers assigned for use with other service providers. This results in a loss of revenue for the original service provider, since the device is, presumably, no longer being used on their network.
From the GSM device manufacturer's point of view, there are other drawbacks to manufacturing devices with service provider locks. For example, manufacturing a device with a particular service provider lock may require the manufacturer to maintain different part numbers for the mobile devices manufactured for the different service providers, since the locking codes and/or algorithms will vary depending on the service provider. This can add to the logistical complexity of manufacturing the device as well as add significant inventory cost.
From the consumers' point of view, most would likely prefer the freedom of purchasing a mobile device without being restricted to one particular service provider. For example, it may be desirable to switch to a different service provider when traveling abroad or to different parts of the country.